Senescent immune cells accumulation promotes brown adipose tissue dysfunction during aging

Brown adipose tissue (BAT)-mediated thermogenesis declines with age. However, the underlying mechanism remains unclear. Here we reveal that bone marrow-derived pro-inflammatory and senescent S100A8+ immune cells, mainly T cells and neutrophils, invade the BAT of male rats and mice during aging. These S100A8+ immune cells, coupled with adipocytes and sympathetic nerves, compromise axonal networks. Mechanistically, these senescent immune cells secrete abundant S100A8 to inhibit adipose RNA-binding motif protein 3 expression. This downregulation results in the dysregulation of axon guidance-related genes, leading to impaired sympathetic innervation and thermogenic function. Xenotransplantation experiments show that human S100A8+ immune cells infiltrate mice BAT and are sufficient to induce aging-like BAT dysfunction. Notably, treatment with S100A8 inhibitor paquinimod rejuvenates BAT axon networks and thermogenic function in aged male mice. Our study suggests that targeting the bone marrow-derived senescent immune cells presents an avenue to improve BAT aging and related metabolic disorders.

Activation of the brown adipose tissue (BAT) promotes non-shivering thermogenesis, conferring beneficial effects on metabolic health. With increasing age, BAT gradually becomes senescent characterized by increased adiposity, inflammatory immune cell infiltration, and reduced thermogenesis, leading to the onset of obesity and age-related metabolic disorders 1,2 . Increasing lines of evidence have demonstrated that aging of the immune system, or immunosenescence, contributes to the aging of solid organs, including white adipose tissues [3][4][5] . However, the role of senescent immune cells in BAT aging remains largely unknown. The sympathetic nervous system (SNS) directly innervates the BAT and plays a key role in driving adipocyte lipolysis, as well as promoting adaptive thermogenesis through the release of noradrenaline from local axons 6,7 . Interactions between the nervous and immune systems have recently emerged as important regulators of adipose tissue function 8 . A neuro-mesenchymal unit is identified to translate neuronal cues into adipose-resident group 2 innate lymphoid cell function, thus modulating host metabolism 9 . Nevertheless, it remains unclear how sympathetic neurons and immune cells cooperate in the BAT to orchestrate adipose metabolism, especially in the context of aging that showed impaired sympathetic innervation 10 . S100A8 belongs to the S100 calcium-binding protein family and has recently gained much interest as a critical alarmin modulating the inflammatory response after releasing from neutrophils and monocytes 11 . Extracellular S100A8 interacts with the pattern recognition receptors Toll-like receptor 4 (TLR4) and advanced glycosylation end-product specific receptor (AGER) promoting cell activation and recruitment 11,12 . Recent evidence using single-cell RNA sequencing (scRNA-seq) identified S100A8 as one of the most commonly increased proteins across tissues including the BAT, in aged rats 13 , raising a potential role of S100A8 in accelerating BAT aging.
In this study, we leveraged a published scRNA-seq database and identified a group of S100A8 + senescent immune cell that accumulated in the BAT of aged mice and drove BAT aging. Specifically, we found that S100A8 + senescent immune cells, coupled with sympathetic neurons and brown adipocytes, established an interface that inhibited sympathetic innervation. Importantly, we demonstrated that the S100A8 inhibitor, paquinimod, diminished S100A8 + immune cells in BAT, therefore ameliorating obesity-associated metabolic dysfunction in aged mice.

Results
Pro-inflammatory and senescent S100A8 + immune cells accumulate in the BAT with age To identify the immune cell populations that contribute to BAT aging, we first reanalyzed the scRNA-seq dataset of BAT from a previous study depicting the aging-associated transcriptional landscape in multiple tissues of young (5 months) and aged (27 months) rats 13 . We found that the pools of T cells and neutrophils were greater in aged rats versus young rats (Fig. 1a). Gene Ontology (GO) analysis revealed that upregulated genes in T cells and neutrophils of aged rats were mostly enriched in the regulation of inflammatory responses and leukocytes cell-cell adhesion, respectively (Fig. 1b). Among the genes upregulated in both T cells and neutrophils in the BAT from the aged rats, we particularly noted a significant elevation in S100a8 expression (Fig. 1c), which plays a prominent role in inflammatory processes, leukocytes chemotaxis, and adhesion 11 . We found that the S100a8 expression in neutrophils was the highest among immune cells (Fig. 1c), while the S100A8 + T cells expanded by about 150 times in aged rats compared to the young rats, followed by S100A8 + neutrophils and macrophages (Fig. 1d). We further performed single-cell transcriptional profiling to identify discrete subclusters. Both CD4 + and CD8 + T cells showed elevated S100a8 expression (Fig. 1e). Within the 9 transcriptionally distinct neutrophil subclusters ( Supplementary Fig. 1a), clusters 1, 2, and 8 contained the most S100A8 + neutrophils in the aged rats, while none of them exist in young rats ( Supplementary Fig. 1b). And subcluster 4 and 9 of divided macrophage subclusters also showed increased S100a8 expression (Fig. 1f, g). Genome-wide transcriptional profiling of S100A8 + T cells, neutrophils, and macrophages showed that S100A8 + T cells, neutrophils, and macrophages exhibited high expression of inflammatory and aging-related genes including S100a8, S100a9, Il1b, Ccl6, Lyz2, Cebpb, Sod2, Lrg1, G0s2, Mmp8, Ccl3 ( Supplementary Fig. 1c). In support of this, S100A8 + T cells, neutrophils, and macrophages correlated well with aged T cells, aged neutrophils, and aged macrophages, respectively (Supplementary Fig. 1d-f), which all showed high expression of p21 ( Supplementary Fig. 1g). By contrast, the expression levels of classical senescence-associated secretory phenotype (SASP) Ifng and Tnf were much lower than that of S100a8 ( Supplementary  Fig. 1h, i). Additionally, we analyzed the transcriptional profile of S100A8 + immune cells from the BAT of young and aged rats. Though some KEGG pathways were shared, we noticed that canonical proinflammatory pathways, such as the IL-17 signaling pathway and TNF signaling pathway, were specifically enriched in the S100A8 + immune cells from the BAT of aged rats, but not that of young rats (Supplementary Fig. 1j). Moreover, the expression of top 10 marker genes of S100A8 + immune cells, which were pro-inflammatory and senescent, were much higher in the BAT of aged rats than that of young rats ( Supplementary Fig. 1k). Thus, we characterized the S100A8 + immune cells as a specific group of pro-inflammatory and senescent immune cells that accumulated in the BAT of rats during aging.
In mice, aged BAT showed increased S100A8 protein levels during aging, accompanied by declined UCP1 and PGC1α expression (Supplementary Fig. 1l). To be noted, the S100A8 protein was enriched in the stromal vascular fractions (SVFs), consisting of immune cells and other non-immune cells, but hardly detectable in the adipocytes of BAT from aged (15-month-old) mice ( Supplementary Fig. 1m). In addition, immunofluorescence staining showed that CD31 rarely costained with S100A8 ( Supplementary Fig. 1n) in the BAT, further supporting that S100A8 + cells were mainly immune cells. We further performed flow cytometry analysis to determine the percentages of S100A8 + immune cells in the BAT of 2-month-, 8-month-, and 15-month-old mice ( Supplementary Fig. 1o, p). The frequencies of S100A8 + CD3 + T cells and S100A8 + CD11b + myeloid cells were both increased in the BAT of aged mice (Fig. 1h, i). Immunostaining also verified that the BAT of aged mice displayed increased accumulation of S100A8 + CD3 + T cells and S100A8 + CD11b + myeloid cells compared to that of younger mice (Fig. 1j, k). However, the protein levels of S100A9, a functionally related protein of S100A8, were relatively lower and showed less obvious changes during aging ( Supplementary Fig. 1l). Flow cytometry showed increased S100A9 + T cells and myeloid cells infiltrating the BAT during aging, but to a much mild degree when compared to S100A8 + immune cells (Fig. 1h, i and Supplementary  Fig. 1q), indicating a more important involvement of S100A8 in BAT aging. These findings supported that S100A8 + immune cells in mice possess similar properties as in rats. Taken together, we have identified that a pro-inflammatory and senescent S100A8 + immune cell subtype accumulated in the BAT of rats and mice with age, which may play an important role in the progression of BAT aging.
BAT-infiltrating S100A8 + immune cells originate from the bone marrow Adipose-infiltrating pro-inflammatory immune cells are usually derived from bone marrow, which interferes with adipose function by secreting inflammatory cytokines 14 . To determine whether BAT-invading S100A8 + immune cells are bone marrow-derived, the S100A8 + immune cells and S100A8immune cells were isolated from the bone marrow of 15-monthold mice by flow cytometry and were stained with lipophilic dye PKH26 for cell-tracing ( Fig. 2a and Supplementary Fig. 2a). Compared with S100A8immune cells, S100A8 + immune cells showed high S100A8 protein levels, confirming the specificity of cell sorting ( Supplementary  Fig. 2b). To be note, increased levels of P16, P21, and phosphorylated histone H2AX and decreased levels of PCNA were observed in S100A8 + immune cells versus S100A8immune cells, indicating increased DNA damage and decreased proliferation activity ( Supplementary Fig. 2b). We also detected increased expression of SASP factors (such as Tnf and Ifng) and senescent markers (such as p16 and p21) in S100A8 + immune cells versus S100A8immune cells ( Supplementary Fig. 2c), further demonstrating S100A8 + immune cells were pro-inflammatory and senescent. Cells were then transferred intravenously into 2-month-old young mice and cell tracking was analyzed 1 week later. The PKH26 red fluorescence indicated that abundant bone marrow-derived S100A8 + immune cells, but not S100A8immune cells, accumulated largely in the BAT, while little in the inguinal adipose tissue (iWAT), epididymal adipose tissue (eWAT), and liver (Fig. 2b). Consistently, we observed increased GFP signaling in the BAT of 15-month-old S100a8-Cre-EGFP reporting mice, while their accumulation in other tissues is much less than that in the BAT (Supplementary Fig. 2d). Similarly, mice transferred with bone marrow-derived GFP + immune cells from 15-month-old S100a8-Cre-EGFP mice showed that GFP + immune cells preferentially infiltrated BAT rather than other tissues ( Supplementary Fig. 2e). To further investigate the bone marrow origin of BAT-infiltrating S100A8 + immune cells, we generated bone marrow chimeric mice by intravenously transferring hematopoietic stem or progenitor cells (HSPCs) (Lin -Sca-1 + c-Kit + ) isolated from S100a8-Cre-EGFP mice into lethally irradiated 2-month-old wild-type (WT) mice (Fig. 2c, d). These S100a8-Cre-EGFP mice-derived HSPCs can differentiate into neutrophils, monocytes, and lymphocytes which are genetically labeled with GFP once they express S100A8. Mice were successfully reconstituted 5 weeks after transplantation. Results suggest that GFP intensity in the BAT increased at 6 months and 12 months post reconstitution (Fig. 2e) Fig. 1 | Pro-inflammatory and senescent S100A8 + immune cells accumulate in brown adipose tissue with age. a Bioinformatics analysis of scRNA-seq of BAT from young (5-month-old) and aged (27-month-old) rats. b GO analysis of differentially expressed genes in T cells (left) and neutrophils (right) from aged rats. c Violin plots for gene expression of S100a8 in cell populations of BAT from young and aged rats. d Relative cell proportion of S100A8 + and S100A8 − cell populations in the BAT of young and aged rats. e Violin plots for gene expression of S100a8 in T cell subclusters of young and aged rats. f UMAP plot shows clustering of macrophages based on gene expression. g Violin plots for gene expression of S100a8 in macrophage subclusters of young and old. h, i Representative flow cytometry plots and quantification of the frequencies of S100A8 + cells in CD45 + CD3 + T cells (h), and CD45 + CD11b + myeloid cells (i), in the stromal vascular fractions (SVFs) of BAT from 2-, 8-and 15-month-old mice (n = 3-5/group). j Representative images of CD3 (green) and S100A8 (red) and staining in the BAT of young (2 months) and aged (15 months) mice. Scale bar, 50 µm. k Representative images of CD11b (green) and S100A8 (red) staining in the BAT of young (2 months) and aged ( GFP + CD3 + T cells and GFP + CD11b + myeloid cells emerged in the BAT 2 months after reconstitution, and increasingly expanded after 6 months and 12 months (Fig. 2f, g). Noticing the preferential BATaccumulation of S100A8 + immune cells, we further tested the expression of CXCR4, CXCR2, and CD62L, important regulators that affect cell migration and trafficking [15][16][17] , in the S100A8 + immune cells. We found that S100A8 + immune cells showed high expression of CXCR2 compared to CXCR4 and CD62L ( Supplementary Fig. 2f). Interestingly, the expression of CXCL1, a ligand of CXCR2, was intensified in the BAT during aging, while its expression showed a much milder increasing  a Schematic diagram for isolation, labeling and transferring bone marrow-derived S100A8 + immune cells into young mice. This diagram was created with BioRender.com. b Representative images of PKH26-labeled S100A8 − or S100A8 + immune cells infiltrating BAT, iWAT, eWAT and liver (n = 3). Scale bar, 100 μm. c Schematic diagram for bone marrow reconstitution of mice with HSPCs isolated from S100a8-Cre-EGFP mice. This diagram was created with BioRender.com. d Gating strategy of bone marrow hematopoietic stem/progenitor cells (HSPCs). e Representative images of GFP + cells in the BAT of bone marrow reconstitution mice during aging (n = 4). Scale bar, 100 μm. f, g Representative flow cytometry plots and quantification of the frequencies of GFP + cells in CD45 + CD3 + T cells (f) (n = 5/group), and CD45 + CD11b + myeloid cells (g) (n = 5/group), in the SVFs of BAT from bone marrow reconstitution mice during aging. Data shown are representative of three independent experiments with similar results. n indicates the number of biologically independent samples examined. Data are shown as mean ± SEM. Statistical differences were supposed to be significant when P < 0.05. Statistical analysis was performed by one-way ANOVA with Tukey's multiple-comparison test (e-g). Source data are provided as a Source Data File. trend in the liver and iWAT ( Supplementary Fig. 2g, h). These results suggest that the CXCL1-CXCR2 axis may be responsible for the mobilization and migration of S100A8 + immune cells to the BAT. Together, these results demonstrate that BAT-infiltrating S100A8 + immune cells originate from the bone marrow.
Senescent S100A8 + immune cells are sufficient to inhibit BAT thermogenic function in young mice To investigate the potential role of senescent S100A8 + immune cells in BAT aging, we analyzed the metabolic phenotype in S100A8 + immune cells transplanted mice as described above (Fig. 2a). Along with the decreased levels of thermogenic genes, as measured by Ucp1 and Ppargc1a (Fig. 3a-c), we also observed increased levels of p16 and p21, the aging markers, in the BAT of mice transplanted with S100A8 + immune cells compared to those transplanted with S100A8 − immune cells (Fig. 3c, d). Indirect calorimetry analysis by Comprehensive Lab Animal Monitoring System (CLAMS) revealed that transferring of S100A8 + immune cells inhibited oxygen consumption and energy expenditure under both room temperature conditions and after cold stimulation, without significant effects on food intake, activities, or respiratory exchange ratio (RER) (Fig. 3e, f and Supplementary  Fig. 3a-c). During acute cold acclimation, mice transferred with
h Hematoxylin-eosin staining and adipocyte cell-diameter quantification for BAT of mice transferred with S100A8 + immune cells or S100A8immune cells. Scale bar, 50 μm. i Relative mRNA levels of thermogenic and aging-related genes in the BAT of mice injected with AAV-Scramble and AAV-ShS100a8 (n = 5). j Core body temperature of mice injected with AAV-Scramble and AAV-ShS100a8 under cold stimulation (n = 5). Data shown are representative of three independent experiments with similar results. n indicates the number of biologically independent samples examined. Data are shown as the mean ± SEM. Statistical differences were supposed to be significant when P < 0.05. Statistical analysis was performed by two-way ANOVA (g, j), ANCOVA with body weight as covariant (e, f) or unpaired two-tailed Student's t test (a, c, d, i). Source data are provided as a Source Data File. S100A8 + immune cells displayed lower core body temperatures (Fig. 3g). Decreased adaptive thermogenesis is usually coupled with decreased lipolysis 18 . In line with this, increased size of lipid droplets was observed in BAT of mice transferred with S100A8 + immune cells (Fig. 3h). Likewise, mice transferred with GFP + immune cells mentioned above ( Supplementary Fig. 2e) also showed impaired thermogenic capacity manifested by decreased protein and mRNA levels of UCP1 and PGC1α ( Supplementary Fig. 3d, e). To be noted, while S100A8 protein levels elevated in the BAT of mice transferring S100A8 + immune cells ( Supplementary Fig. 3f), serum S100A8 levels were not obviously changed during aging ( Supplementary Fig. 3g). We supposed that BAT-infiltrating S100A8 + immune cells and their locallyreleased S100A8 protein, rather than serum S100A8 protein, contributed to BAT dysfunction. We, therefore, performed knockdown of S100a8 gene expression in the BAT of 15-month-old mice by fat-pat injection of adeno-associated viruses (AAV)-delivered short hairpin RNA (ShRNA) targeting S100a8 (AAV-ShS100a8) ( Supplementary  Fig. 3h). Mice with BAT-specific S100a8 deficiency displayed decreased BAT aging markers, increased thermogenic gene expression, and improved cold acclimation compared to those treated with an AAVmediated scrambled shRNA control (Fig. 3i, j). Together, these data demonstrated that the BAT-infiltrating S100A8 + immune cells secreting abundant S100A8 protein were sufficient to impair thermogenic function in young mice.

S100A8 + immune cells, coupling with sympathetic nerves and adipocytes, form neuroimmune adipose interfaces that inhibit sympathetic innervation
We next tried to dissect the mechanism by which S100A8 + immune cells regulated energy expenditure. As a major factor produced by S100A8 + immune cells, S100A8 mainly functions vias TLR4 and AGER 11,19 , the latter of which is lowly-expressed in BAT and immune organs ( Supplementary Fig. 4a). In addition, TLR4 is highly expressed on brown adipocytes and immune cells, but not non-immune SVFs of BAT ( Supplementary Fig. 4b), which is line with the previous findings 20 . We thus chose TLR4 for further study. WT and Tlr4 knockout (Tlr4 −/− ) mice were treated with recombinant S100A8 protein twice a week for 4 weeks. Among the WT mice, S100A8 protein treatment showed no obvious effects on body weight, serum inflammatory cytokines TNF and IL-6, as well as fasting plasma glucose (Supplementary Fig. 4c-e). It is probably due to the low dosage and administration frequency of S100A8 protein treatment. However, like transferring S100A8 + immune cells, WT mice treated with S100A8 recombinant protein showed decreased UCP1 and PGC1α expression ( Supplementary  Fig. 4f). By contrast, Tlr4 deficiency abolished the suppressive effects of S100A8 on BAT thermogenic function ( Supplementary Fig. 4f, g), suggesting TLR4 mediated the inhibitory effects of S100A8 on thermogenesis. To investigate whether TLR4 on immune cells or brown adipocytes is responsible for the regulation of thermogenesis mediated by S100A8, we generated bone marrow chimera mice by transferring bone marrow from Tlr4 −/− mice into recipient WT mice (Tlr4 −/ − BM-WT) and then treated with S100A8 protein. Notably, even though TLR4 in immune cells was ablated in bone marrow chimeric mice, S100A8 treatment still impaired BAT thermogenesis capacity as shown by decreased Ucp1 and Ppargc1a expression ( Supplementary Fig. 4h). We further generated adipocytes-specific Tlr4 knockout mice (Tlr4 fko ) by crossing Tlr4 f/f mice with Adpn-Cre mice. We found that adipocytespecific TLR4 deficiency blocked the effects of S100A8 on the expression of Ucp1 and Ppargc1a ( Supplementary Fig. 4h). These results together suggested that S100A8 may regulate BAT thermogenic function via acting through TLR4 on brown adipocytes but not immune cells. Of note, the levels of thermogenic genes were not disturbed in S100A8-treated brown adipocytes in vitro but marked decreased after treatment of other TLR4 agonists, lipopolysaccharide (LPS), and S100A9 ( Supplementary Fig. 4i, j). To depict the exact mechanism, brown adipocytes were treated with S100A8 recombinant protein and subjected to RNA-seq. A total of 3568 differentially expressed genes (DEGs) with at least a 1.3-fold change were identified, including 1754 genes upregulated and 1814 genes downregulated (Fig. 4a). GO analysis revealed that DEGs were enriched in a series of biological processes, among which multiple neuron function-related pathways including regulation of neuron projection, neuron apoptotic process, and neuronal synaptic plasticity were affected and piqued our interest (Fig. 4b). Sympathetic neurons play essential roles in the BAT thermogenic function via releasing norepinephrine 21 . Tyrosine hydroxylase (TH), a key marker of sympathetic innervation, as well as βtubulin isotype 3A (TUBB3), a neural-specific tubulin, distributed around adipocytes in the BAT of young mice and decreased during aging ( Supplementary Fig. 4k). Importantly, we found that S100A8 + immune cells, which increased during aging, frequently localized adjacent to TH + and TUBB3 + sympathetic nerves in the BAT of 15month-old mice (Fig. 4c, d and Supplementary Fig. 4l). Along with the increased age-associated accumulation of S100A8 + immune cells, the immunofluorescence intensity ofTH was negatively associated with that of S100A8 (Fig. 4e). The spatially close association may indicate a strong functional communication within the three cell types. In mice transferred with S100A8 + immune cell, the sympathetic innervation as indicated by expression of TH, (Ser40) phospho-TH(p-TH), and TUBB3 was impaired ( Fig. 4f and Supplementary Fig. 4m). These results indicated infiltrating S100A8 + immune cells, coupled with sympathetic nerves and adipocytes, form a neuroimmune interface that compromises BAT axonal networks (Fig. 4g).

S100A8 decreases RBM3 expression in brown adipocytes, which ablates sympathetic innervation
To find the direct targets regulated by S100A8, we generated an RNAseq dataset from differentiated brown adipocytes treated with S100A8 recombinant protein or PBS for 24 h and cross-analyzed our RNA-seq dataset with the dataset from BAT of aged mice (GSE25324), thus identified 22 candidate genes (Fig. 4h). Given the sensitivity of TLR4 for possible endotoxin contamination, we reconfirmed the mRNA levels of the candidate genes in brown adipocytes treated with PBS, LPS or S100A8 protein. The results showed that some genes shared similar variation trends after stimulation by LPS or S100A8 protein (Heg1, Impdh1), however, the expression profile of most downstream genes differed (Gbp2, Rbm3, Tor3a, Irf1, Zfp182, Polm, Tmem47) (Supplementary Fig. 5a). Therefore, we suggest that the generation of transcriptional targets were not disturbed by endotoxin contamination. Among the candidates, Rbm3, which is induced by cold shock and has a neuroprotective effect 22,23 , caught our attention. We found that levels of Rbm3 in the BAT, enriched in adipocytes, were induced by cold and decreased during aging ( Fig. 4i and Supplementary Fig. 5b-e). Transferring of S100A8 + immune cells and S100A8 protein treatment reduced the levels of Rbm3 in vivo and in vitro, respectively ( Fig. 4j and Supplementary Fig. 5f, g). Consistent with an in vivo role of TLR4 in mediating the function of S100A8 + immune cells, TLR4 inhibitor TAK-242 treatment reversed the inhibitory effect of S100A8 on RBM3 expression in brown adipocytes ( Supplementary Fig. 5f). It has been reported that S100A8 treatment rapidly induces phosphorylation of p38 and ERK1/2 downstream of TLR4 in target cells 24 . We found that S100A8 induced TLR4-downstream p38 and ERK1/2 phosphorylation in brown adipocytes, and only p38 inhibitor SB203580 but not ERK1/2 inhibitor PD98059 restored RBM3 expression suppressed by S100A8 ( Supplementary Fig. 5h). These results indicate that S100A8 regulated RBM3 expression in a TLR4-p38 axis-dependent manner.
To determine the role of RBM3 in BAT function, we specifically depleted Rbm3 in the BAT of 2-month-old mice via fat-pad injection of Fabp4 promoter-driven AAV delivered shRNA targeting Rbm3 (AAV-Fabp4-ShRbm3, hereafter referred to as AAV-ShRbm3), with a group injected with scrambled control (AAV-Fabp4-Scramble, hereafter Tubulin RBM3 S100A8immune cells S100A8 + immune cells referred to as AAV-Scramble). AAV-ShRbm3 delivery achieved a specific knockdown of RBM3 expression in the BAT but not in the white adipose tissues (Supplementary Fig. 6a). AAV-ShRbm3 mice displayed decreased sympathetic innervation as indicated by TH, p-TH, and TUBB3 (Fig. 4k, l and Supplementary Fig. 6b), accompanied by impaired adaptive thermogenic function manifested as reduced levels of thermogenic genes ( Fig. 4l and Supplementary Fig. 6c) and lower body temperature during acute cold acclimation (Fig. 4m). Moreover, at the age of 6 months, expression of p16 and p21 increased in BAT of AAV-ShRbm3 mice (Supplementary Fig. 6d). These results suggested that Rbm3 deficiency in BAT led to disrupted energy dissipation and accelerated BAT aging.
Overexpression of RBM3 in the BAT alleviates S100A8 + immune cells-induced BAT aging We further specifically overexpressed RBM3 in the BAT by fat-pad injection of Fabp4 promoter-driven AAV expressing Rbm3 into 15month-old mice (hereafter referred to as AAV-RBM3 mice). AAV-RBM3 mice showed specific RBM3 overexpression in the BAT but no other tissues ( Supplementary Fig. 6e). AAV-RBM3 mice displayed enhanced Arbitrary Units S100A8 + -AAV-RBM3 S100A8 + -AAV-NC S100A8 + -AAV-RBM3 S100A8 + -AAV-NC S100A8 + -AAV-RBM3 S100A8 + -AAV-NC S100A8 + -AAV-RBM3 S100A8 + -AAV-NC S100A8 + -AAV-RBM3  Metabolic phenotype levels of TH, p-TH, TUBB3, as well as thermogenic genes such as Ucp1 and Ppargc1a (Supplementary Fig. 6f, g). Meanwhile, expression of aging markers p16 and p21 in the BAT were both suppressed by RBM3 overexpression (Supplementary Fig. 6g). Together, these results demonstrated that RBM3 improved BAT sympathetic innervation and thermogenic function. We next set out to determine whether RBM3 overexpression could rescue the effects of S100A8 + immune cells on BAT function. To achieve this, 2-month-old mice were overexpressed with RBM3 in the BAT via fat-pad injection of AAV-RBM3 and transferred with S100A8 + immune cells (Fig. 5a). Results showed that RBM3 overexpression in the BAT largely prevented impairment of sympathetic innervation and thermogenic capacity caused by S100A8 + immune cells (Fig. 5b-d and Supplementary Fig. 6h). Lower p16 and p21 levels in the BAT were also observed after RBM3 overexpression, indicating a delayed BAT aging process (Fig. 5d). Indirect calorimetry analysis suggested that RBM3 overexpression restored the oxygen consumption levels, without significant effects on food intake or activities ( Fig. 5e and Supplementary Fig. 6i, j). Consistently, RBM3 overexpression endowed S100A8 + immune cell-transferred mice with increased resistance to cold challenge as manifested by a higher core body temperature compared to the control mice (Fig. 5f). Together, these data suggest that the downregulated adipose RBM3 mediates the suppressive effects of S100A8 + immune cells on BAT sympathetic innervation and thermogenic function.

RBM3 in brown adipocytes regulates sympathetic innervation via decreasing the mRNA stability of genes involved in axon guidance
We next investigated the mechanism by which RRM3 regulates sympathetic innervation. RBM3 is reported as a critical RNA binding protein that regulates gene expression 25,26 . To identify the direct target genes bound and regulated by RBM3, we further performed RNA immunoprecipitation sequencing (RIP-seq) in brown adipocytes and RNA-seq of WT and Rbm3-deficient brown adipocytes. 4349 RBM3binding peaks, approximately 77.54% of which were distributed in introns ( Supplementary Fig. 6k), and 2985 DEGs were screened out (Fig. 5g). By cross-analyzed the RIP-seq dataset with the RNA-seq dataset, we identified 909 DEGs with direct RBM3-binding sites (Fig. 5g). GO analysis of the overlapping genes showed that they are mainly enriched in pathways such as focal adhesion, endocytosis, platelet activation, axon guidance, actin cytoskeleton (Fig. 5h). Among them, we paid our attention to axon guidance, a pathway closely related to sympathetic innervation. Within the 14 axon guidancerelated genes, the mRNA levels of Nrp1 and Epha7 decreased while other candidate genes demonstrated only mild or no significant changes in the Rbm3-deficient brown adipocytes (Fig. 5i, j and Supplementary Fig. 6l). Concordantly, the protein levels of NRP1 and EPHA7 were both reduced after knockdown of Rbm3 (Fig. 5k). RBM3 has been reported to function as an RNA chaperone to maintain RNA stability 27,28 . Sequence analysis showed the existence of specific RBM3binding sites on the 3´untranslated region (3´UTR) of Nrp1 and Epha7, which was confirmed by RNA immunoprecipitation (RIP) analysis (Fig. 5l). Importantly, RBM3 knockdown reduced the mRNA stability of Nrp1 and Epha7 (Fig. 5m), suggesting that RBM3 may function through stabilizing the mRNAs of a specific set of key axon guidance genes. To further determine whether these factors were responsible for the effects of RBM3 in the regulation of sympathetic innervation, we performed the knockdown of Nrp1 or Epha7 in RBM3 overexpressed mature brown adipocytes. These adipocytes were then directly cocultured with the PC12 cells, a classical sympathetic neuronal cell 29 . Coculture results suggested that both Nrp1 or Epha7 knockdown abolished the effects of RBM3 on promoting neurite outgrowth ( Fig. 5n and Supplementary Fig. 6m). Together, these results demonstrated that RBM3 in brown adipocytes regulates sympathetic innervation via modulating genes involved in axon guidance.

Transplantation of human S100A8 + immune cells induces BAT aging and thermogenic decline in mice
We next examined the association of S100A8 + immune cells with aging in humans. As adipose-infiltrating immune cells mainly come from circulation, we thus collected peripheral blood cells from young (aged from 16 to 32) and older people (aged 56-69) for further study. Results demonstrated a significant increase of S100A8 + immune cells including S100A8 + T cells in older individuals, compared to young people (Fig. 6a, b). Moreover, S100A8 mRNA levels showed a positive correlation with p21 gene (Kras) expression in human whole blood cells (Fig. 6c), supporting a correlation of human S100A8 with aging. To test whether accumulated human S100A8 + immune cells could cause BAT aging, isolated human S100A8 + immune cells or S100A8 − immune cells were transferred to 2-month-old immuno-deficiency NOD-SCID (nonobese diabetic/severe combined immune-deficient) mice to establish a humanized-mouse model (Fig. 6d). We found that two groups showed similar levels of serum inflammatory cytokines 2 weeks after cell transfer (Fig. 6e), thus excluding the possibility of graft-versus-host disease. Flow cytometry analysis and immunostaining of human CD45 (a marker of immune cells) suggested that transferred human S100A8 + immune cells infiltrated mice BAT in 2 weeks (Fig. 6f, g). Of note, mice transferred with human S100A8 + immune cell-transferred showed lower TH expression in the BAT than that of mice transferred with S100A8immune cells (Fig. 6g). These results further support our hypothesis that the established neuroimmune adipose interface would subdue sympathetic innervation. Though the percentages of S100A8 + cells were much less in humanized mice than that in aged mice, S100A8 + immune cell-transferred mice exhibited decreased UCP1 expression and increased P16 and P21 expression, accompanied by increased β-gal staining (Fig. 6h, i). Furthermore, human S100A8 + Fig. 5 | Adipose RBM3 mediates the action of S100A8 + immune cells in sympathetic innervation. a Schematic diagram of experimental processes. This diagram was created with BioRender.com. b Representative images of TH staining in the BAT of mice injected with AAV-RBM3 or AAV-NC and transferred with S100A8 + immune cells. Scale bar, 100 µm. c Representative immunoblots of UCP1, TH, p-TH, TUBB3 and RBM3 in the BAT of mice injected with AAV-RBM3 or AAV-NC and transferred with S100A8 + immune cells (n = 3). d Relative mRNA levels of Rbm3, Th Ucp1, Ppargc1α, p16, and p21 in the BAT of mice injected with AAV-RBM3 or AAV-NC and transferred with S100A8 + immune cells (n = 5). e Oxygen consumption of mice injected with AAV-RBM3 or AAV-NC and transferred with S100A8 + immune cells or S100A8immune cells (n = 6-7). f Core body temperature of mice injected with AAV-RBM3 or AAV-NC were transferred with S100A8 + immune cells and subjected to cold challenge (n = 5). g Venn diagram showing overlapped genes which are RBM3-bounded and differentially expressed upon Rbm3 knockdown. h KEGG analysis of the 909 overlapped genes shown in (g). i Heatmap of axon guidancerelated gene expressions based on RNA-seq data of Rbm3 knockdown differentiated brown adipocytes. j Relative mRNA levels of Rbm3 and axon guidance-related genes Nrp1 and Epha7 in differentiated brown adipocytes transfected with si-Rbm3 or si-NC (n = 3). k Representative immunoblots of NRP1 and EPHA7 in differentiated brown adipocytes transfected with si-Rbm3 or si-NC (n = 4). l RNA immunoprecipitation (RIP) assay assessing RBM3 binding on 3´UTR of Nrp1 and Epha7 in differentiated brown adipocytes (n = 4). m Relative mRNA level of Nrp1 and Epha7 in si-Rbm3 or si-NC transfected brown adipocytes upon transcriptional inhibition with actinomycin D at indicated time (n = 3). n Representative images of TUBB3 staining in PC12 cells cocultured with differentiated brown adipocytes overexpressed with RBM3 and transfected with si-Nrp1 or si-Epha7 (n = 4). Scale bar, 100 µm. Data shown are representative of three independent experiments with similar results. n indicates the number of biologically independent samples examined. Data are shown as the mean ± SEM. Statistical differences were supposed to be significant when P < 0.05. Statistical analysis was performed by one-way ANOVA with Tukey's multiple-comparison test (n) or two-way ANOVA (f, m), ANCOVA with body weight as covariant (e) or unpaired two-tailed Student's t test (c, d, j-l). Source data are provided as a Source Data File.
immune cell-transferred mice showed decreased BAT thermogenesis and core body temperature after cold stimulation (Fig. 6j, k). Together, these results demonstrated a striking contribution of human S100A8 + immune cells in inducing aging-like BAT dysfunction.
Paquinimod disrupts the function of S100A8 + senescent immune cells and ameliorates age-related metabolic dysfunction Given S100A8 functioned via TLR4, we next determine whether pharmacological blocking S100A8 binding to TLR4 would delay BAT aging. Paquinimod is a selective inhibitor of S100A8/A9 by preventing S100A8/A9 binding to TLR4 30 . 15-month-old WT mice were administrated with paquinimod or vehicle for 4 weeks. We found that paquinimod treatment decreased BAT infiltration of S100A8 + T cells and myeloid cells (Fig. 7a, b). In line with the diminished S100A8 + immune cells infiltration, paquinimod restored the sympathetic innervation and thermogenic function in BAT (Fig. 7c-e), and delayed the aging process of BAT, marked by decreased levels of P16 and P21 (Fig. 7f). BAT thermogenesis is critical for whole body energy homeostasis. To determine whether paquinimod could ameliorate age-  related metabolic dysfunction, normal chow diet (NCD)-fed 15-monthold mice were administrated with paquinimod for 6 months. Without affecting food intake, paquinimod treatment decreased body weight (Fig. 7g, h) and improved fasting glucose levels (Fig. 7i). Exposure to high-fat diet (HFD) exacerbates age-related thermogenic deficits. We found that HFD-fed mice treated with paquinimod showed much less weight gain and ameliorated metabolic phenotype including improved glucose tolerance, insulin sensitivity, decreased eWAT and liver mass, smaller adipocyte sizes, and reduced liver lipid accumulation ( Fig. 7j-n). These results demonstrate that by targeting S100A8 binding to TLR4, paquinimod improves BAT sympathetic innervation and enhanced thermogenic function, leading to metabolic homeostasis.

Discussion
The peripheral nervous system directly innervates local tissues and contributes to the physiology or pathology of tissue biology 31 . Accumulating evidence has suggested that integration of the signals from sympathetic neurons and immune cells is critical for adipose tissue function 6,7,9,32 , strongly indicating a close interaction among these three cell types. While most studies focus on the immune cells modulated by sympathetic tone to orchestrate adipose metabolic homeostasis, the immune regulation of sympathetic innervation remains to be fully determined, especially during the aging process. In this study, we found abundant bone marrow-derived S100A8 + immune cells invading the BAT of older animals and identified a neuroimmune adipose interface that controls BAT innervation.
Senescent immune cells accumulate in multiple tissues with obesity and age [33][34][35] and secrete a variety of pro-inflammatory cytokines, chemokines, and proteases, termed the SASP 34 . We have previously demonstrated that senescent GCA + macrophages and neutrophils in bone marrow caused skeletal aging 36 . Here, we identified that senescent immune cells accumulated in the BAT were S100A8 + T cells, neutrophils, and, to a lesser extent, macrophages. These results were in concordance with an age-associated increase of S100A8 in the BAT and serum of aged rats 13 . Though neutrophils and macrophages were thought to be the major producers of S100A8, unexpectedly, we found that S100A8 + T cells changed the most with age, which indicates that T cells rather than other immune cells, may exert a more profound influence on BAT aging. Indeed, our results are consistent with previous studies that increased the proportion of T cells accumulated in the BAT of older mice 37 . While the authors of that study suggested that senescent T cells secrete IFN-γ to induce BAT aging, we did not observe significant changes in IFN-γ or TNF-α production in the BAT-resident T cells or other cell types in aged rats. Together, we conclude that S100A8 + immune cells function as the dominant immune subtypes inducing BAT aging.
It is suggested that CXCR2 is critical for the mobilization of neutrophils from the bone marrow 38 . Here, we found that CXCR2 was abundantly expressed in S100A8 + immune cells. Results also showed that the expression levels of CXCL1, the ligand of CXCR2, was obviously magnified in the BAT after aging, while less upregulated in the liver or iWAT. Thus, we speculate that the CXCL1-CXCR2 axis may be responsible for the preferential BAT distribution, which awaits further investigation. The CXCL12-CXCR4 axis was involved in the retention of mature neutrophils in the bone marrow 39 . As aging leads to decreased CXCL12 expression in the bone marrow 40 , it is possible that aging defects in the bone marrow may also prevent senescent immune cells from homing into the bone marrow efficiently and contribute to the accumulation of S100A8 + immune cells in peripheral tissues including the BAT. Consistent with this, the numbers of neutrophils prominently increased during aging 13 .
We and others have demonstrated that adipose-resident cells in the BAT also regulate sympathetic innervation 2,6,7,21,41 . S100A8 inhibited cold-induced thermogenesis, which was consistent with a previous report 42 . However, we found that S100A8 had no direct effects on thermogenic gene expression, but rather suppressed genes involved in axon guidance in brown adipocytes. S100A8 may act on tissue-resident macrophages to amplify the inflammatory response, which may further disturb thermogenesis 43 . However, we found that adipocytespecific Tlr4 knockdown but not Tlr4-deficiency in immune cells abolished the inhibitory effects of S100A8 on sympathetic innervation and thermogenesis. Together, we demonstrated that S100A8 mainly acts on adipocytes to regulate sympathetic innervation and thermogenic capacity thus driving BAT aging rather than inducing inflammation.
It is reported that RBM3 plays diverse physiological and pathological roles in inflammation, neural plasticity, stem cell properties, and cancer development [44][45][46][47] . Of note, RBM3 is increased in the muscle of long-lived animals and promotes skeletal muscle hypertrophy by enhancing mRNA stability and translation 48 . We found that RBM3 was gradually decreased in the BAT upon aging and that S100A8 treatment decreased RBM3 expression via activating p38 signaling. As RBM3 provides a neuroprotective effect via blocking the activation of p38 signaling in neuroblastoma cells 49 , it is tempting to speculate that overexpression of RBM3 would abolish the neurotoxicity effect of S100A8. Indeed, we found that the reduced sympathetic innervation and thermogenic function induced by S100A8 were restored by RBM3 overexpression, highlighting a critical role of the S100A8-RBM3 axis in regulating sympathetic innervation.
Senescent cells are emerging targets for diseases during aging 50 . Besides eliminating senescent cells, targeting the SASP of senescent cells may be a promising strategy to delay aging. Paquinimod is reported as a potent inhibitor of insulitis and diabetes development in the NOD mouse 30 . Results from preclinical studies suggest a promising effect of paquinimod in treating human lupus 51 . Recently, paquinimod is shown to rescue pneumonia with a substantial reduction of viral loads in SARS-CoV-2-infected mice 52 . Here, as a specific compound preventing S100A8 binding to TLR4, we expanded the function of paquinimod into restoring sympathetic innervation. Therefore, it is likely that paquinimod would be a hopeful drug for rejuvenating BAT function and metabolic homeostasis during aging.
Taken together, this study reveals that the bone marrow-derived S100A8 + immune cells play a key role in driving BAT aging via inhibiting sympathetic innervation and that targeting S100A8 + immune cells offer a potential treatment strategy for age-related metabolic disorders. Fig. 6 | Transplantation of human S100A8 + immune cells induce BAT aging and thermogenic decline in mice. a, b Representative flow cytometry plots and quantification of frequencies of S100A8 + immune cells (a) and T cells (b) in peripheral blood cells from young and old normal healthy donors (all males; Young: n = 9, Old: n = 13). c Correlation of S100a8 with p21 (Kras) expression in human whole blood cells based on data from the Genotype Tissue Expression (GTEx) database. d Schematic diagram of experimental processes of transplantation of human S100A8 + or S100A8 − immune cells into NOD-SCID mice. This diagram was created with BioRender.com. e Serum concentration of TNF and IL-6 in transplanted mice (n = 5). f Representative flow cytometry plots and quantification of frequencies of human CD45 + immune cells in the SVFs of BAT from transplanted mice (n = 5). g Representative images of TH and human CD45 staining in the BAT of transplanted mice. Scale bar, 100 μm. h Representative immunoblots of UCP1, P16, and P21 in the BAT of transplanted mice (n = 5). i SA-β-gal staining of BAT and its frozen sections of transplanted mice. j Representative infrared thermal images of BAT of transplanted mice subjected to cold stimulation for 6 h (n = 5). k Core body temperature of transplanted mice under cold stimulation (n = 5). Data shown are representative of three independent experiments with similar results. n indicates the number of biologically independent samples examined. Data are shown as the mean ± SEM. Statistical differences were supposed to be significant when P < 0.05. Statistical analysis was performed by two-way ANOVA (k), or unpaired two-tailed Student's t test (a, b, e-h). Source data are provided as a Source Data File.

Animals
The S100a8-Cre-EGFP mice and Tlr4-knockout mice (Tlr4 −/− ) were purchased from Cyagen (China Bioinformatics analysis of single-cell RNA-seq (1) Primary analysis of raw read data. Raw reads (GEO: GSM4331816, GSM43318173) were processed with fastQC and fastp to remove lowquality reads. Poly-A tails and adaptor sequences were removed by cutadapt. After quality control, reads were mapped to the reference genome Rnor_6.0 using STAR. Gene counts and UMI counts were acquired by featureCounts software. Expression matrix files for subsequent analyses were generated based on gene counts and UMI counts (Singleron Biotechnologies, Nanjing, China). (2) Quality control, dimension-reduction and clustering. Cells were filtered by gene counts below 500. Cells with over 10% mitochondrial content were removed.

Intra-BAT injection of adeno-associated virus
Recombinant adeno-associated serotype 8 viruses with Fabp4 promoter for RBM3 overexpression in adipocytes (AAV-Fabp4-RBM3) or RBM3 knockdown in adipocytes (AAV-Fabp4-ShRbm3) and S100A8 knockdown (AAV-ShS100a8) were all purchased from GeneChem Company (Shanghai, China). AAV-Fabp4-RBM3 and AAV-ShS100a8 were injected in situ into the BAT of mice of 15-month-old, while AAV-ShRbm3 was injected into the BAT of mice of 2-month-old, all at the concentration of 5 × 10 10 vg per side of BAT. The control group was injected with AAV-NC (AAV-Fabp4-RBM3) or AAV-Scramble (AAV-Fabp4-ShRbm3 or AAV-ShS100a8). The procedure of in situ injection is as follows. The anesthetized mice were placed on the workbench, and a 2 cm surgical incision was cut on the back of the mice after shaving and disinfection. Carefully separate the fascia and white adipose tissue around the BAT under the skin, and inject 3-4 points of the BAT on both sides with a micro sampler. After completion, the skin at the edge of the surgical incision was disinfected with compound iodine, sutured, and then disinfected again.
Adoptive transfer of S100A8 + immune cells and GFP + immune cells Bone marrow cells from 15-month-old mice or 15-month-old S100A8-Cre-EGFP mice were collected by flow cytometry. Cells were respectively stained with anti-mouse/human S100A8 antibody for 60 min at 4°C and then stained with Alexa fluor 647-anti-rabbit IgG, APC-Cy7anti-mouse CD45, FITC-anti-human/mouse CD11b and Percp-antimouse CD3 for 30 min at 4°C. For S100A8-Cre-EGFP mice, cells were directly stained with antibodies for CD45, CD11b and CD3. Stained cells were immediately sorted by BD FACS Diva software v9.0 and flow cytometry Aria (BD Biosciences), routinely to more than 95% purity. Isolated bone marrow S100A8 + immune cells and corresponding S100A8immune cells were transferred into 2-month-old male mice intravenously (5 × 10 6 cell /300 μl/mice). The transplanted mice were euthanized 4 weeks later. As for human S100A8 + immune Cells, peripheral blood cells were obtained from older people, and erythrocytes were lysed. Cells were stained with anti-mouse/human S100A8 antibody (Proteintech, 15792-1-AP) for 60 min at 4°C and then stained with Alexa fluor 647-anti-rabbit IgG, FITC-anti-human/mouse-CD11b, and Percp-anti-human CD3 for 30 min at 4°C. About 2 × 10 6 human S100A8 + immune cells were transferred into NOD-SCID mice intravenously. NOD-SCID recipient mice were euthanized 2 weeks later. For cell tracing, isolated S100A8 + immune cells were stained with PKH26 (Sigma, CGLDIL) before transfer, and mice were euthanized 1 week later. Otherwise, isolated S100A8 + immune cells were directly transferred into recipient mice whose metabolic phenotype was analyzed at indicated time points.

Bone marrow chimeric mice construction
For the HSPCs transplantation experiment, bone marrow cells were collected from 2-month-old S100a8-Cre-EGFP mice by flow cytometry. The cells were respectively stained with Zombie Aqua™ dye for 15 min at room temperature and then stained with FITC-antimouse Lineage cocktail, BV412-anti-mouse c-Kit, and APC antimouse Sca-1 for 30 min at 4°C. The Linagec-Kit + sca-1 + HSPCs were immediately sorted by flow cytometry Aria (BD Biosciences), routinely to more than 95% purity. About 1 × 10 5 isolated HSPCs together with additional 2 × 10 5 mature bone marrow cells (to ensure the survival of the recipient mouse shortly after myeloablative) were intravenously transferred into lethally irradiated (950 cGy, delivered in split dose 3 h apart) recipient WT mice within 24 h. Multilineage peripheral blood reconstitution was determined 5 weeks after transplantation by flow cytometry analysis of blood samples using a cocktail of CD45, CD11b, Ly6G, Ly6C, CD3ε, and CD19. For Tlr4 −/− bone marrow transplantation, about 1 × 10 7 bone marrow cells from Tlr4 −/− mice were intravenously transferred into lethally irradiated recipient WT mice within 24 h. The reconstitution effect was detected 8 weeks after transplantation.

In vivo protein and compound treatments
For recombinant S100A8 protein treatment, S100A8 protein (SinoBiological, 50228-M07E) was dissolved in PBS and administrated by tail vein injection at the dose of 2.5 μg per mouse twice a week for 4 weeks. For paquinimod treatment, paquinimod (Med-Chem Express, HY-100442) was sequentially dissolved in 2% DMSO, 30% PEG300, 5% Tween80, and ddH 2 O and mice were administrated at the concentration of 1 mg/kg/day by oral gavage three times a week as indicated. The control group was given an equal volume of normal saline. Under ND feeding, 15-month-old male mice were treated with paquinimod and vehicle for 4 weeks or 6 months. Under HFD feeding, 15-month-old male mice were treated with paquinimod and vehicle for 12 weeks. The weights of mice were monitored weekly.

Glucose tolerance test (GTT) and insulin tolerance test (ITT)
For glucose tolerance tests (GTTs), mice were fasted overnight and treated with an intraperitoneal injection of glucose (1 g/kg). For insulin tolerance tests (ITTs), mice were intraperitoneally treated with insulin (0.75U/kg) after 6 h fasting. Tail venous blood was collected at 0, 15, 30, 60, 90, and 120 min after glucose or insulin injection, and blood glucose levels were measured using the glucometer.

Cold exposure studies
Mice were single-housed in cages without bedding at 6°C with free access to food and water. Core body temperature was monitored hourly. At the end of the experiment, mice were euthanized and fat tissues were isolated to determine thermogenic gene and protein expression.

Comprehensive lab animal monitoring system
Indirect calorimetry experiments were conducted with a Comprehensive Lab Animal Monitoring System (CLAMS, Columbus Instruments, USA). Mice in the CLAMS were fed ad libitum and maintained at either room temperature (25°C) or cold (4°C) conditions for a total of 3 days. Results of oxygen consumption, activity, and RER were collected and analyzed.

Differentiation and coculture of PC12 cells with brown adipocytes
PC12 cells (Procell, CL-0480) were maintained in RPMI-1640 (Gibco) supplemented with 10% FBS (Gibco) and 1% penicillin/streptomycin solution (Solarbio). C3H10T1/2 cells were transfected with RBM3expression or vehicle plasmid and differentiated into mature brown adipocytes. The mature brown adipocytes were then transfected with si-Nrp1 or si-Epha7 to knockdown Nrp1 or Epha7. For coculture of PC12 cells with brown adipocytes, 4 × 10 4 mature brown adipocytes treated as above and 8 × 10 3 PC12 cells were seeded onto coverslips in 24-well plates. To induce PC12 cell differentiation, cells were treated with 50 ng/ml NGF (Peprotech) for 5 days in low-serum medium (2% FBS) and the medium was replaced every 2 days. After 5-day induction, cells were fixed with 4% paraformaldehyde and proceeded to perform immunofluorescence staining to determine neurite outgrowth of PC12 cells. Images were representative of three independent experiments. Neurite lengths were measured from a total of 4~7 wells per group by Image J.

Flow cytometry analysis
BAT SVFs were isolated as previously described 2 . Briefly, freshly dissected brown adipose tissues were finely minced (1-2 mm pieces) and incubated in DMEM containing 0.15% collagenase II (Sigma, C6885) at 37°C with thoroughly shaking for 10 min. The harvested cells were centrifuged at 3000×g for 5 min, resuspended with 1% FBS-PBS and filtered through a 100 μm strainer. Bone marrow cells were flushed from mice tibias and femurs with a 1 ml syringe using ice-cold PBS containing 1% PBS, and erythrocytes were lysed. Isolated SVFs and bone marrow cells were first incubated with Zombie Aqua™ Fixable Dye to assess the live or dead status of cells. To detect the expression of surface molecules, cells were first incubated with anti-mouse CD16/ 32 to reduce nonspecific binding of antibodies, followed by incubation with the indicated antibodies for 60 min (S100A8 and S100A9 antibody) or 20- qRT-PCR was performed as previously reported 2 . Briefly, total RNAs were extracted from tissue or cultured cells using the Trizol reagent (Invitrogen). mRNAs were reverse-transcribed and then amplified using a real-time PCR system (Applied Biosystems). The primer sequences used for real-time PCR are listed in Supplementary  Table S1.

Statistical analyses
All experiments were performed at least three times. The data are expressed as mean ± SEM. Two-tailed Student's t test was used to compare the two groups. When comparing the difference between multiple groups, one-way ANOVA with Tukey's multiple comparison test or two-way ANOVA were applied. The Analysis of Covariance (ANCOVA) test was performed to analyze differences in oxygen consumption and energy expenditure between groups while statistically controlling for the effects of covariate body weight. Statistical differences were supposed to be significant when P < 0.05. The analysis was conducted using GraphPad 8.0 software. To analyze the correlation between the immunofluorescence level of TH and S100A8, Pearson's correlation test was applied.

Reporting summary
Further information on research design is available in the Nature Portfolio Reporting Summary linked to this article.

Data availability
The RNA-Seq data and RIP-Seq data produced in this paper have been deposited in the Sequence Read Archive database with the BioProject accession number PRJNA852952. The accession numbers for published dataset used in this paper are GSM4331816, GSM4331817 and GSE25324. The data for Genotype Tissue Expression (GTEx) database are acquired from GEPIA2. The Tlr4 and Ager expression profiles are acquired from BioGPS. All other data supporting the findings of this study are available within the paper and its Supplementary Information. Source data are provided with this paper.